What does everyone mean by “Nipple Stimulation”

Destynee • PCOS • 2 natural born • pregnant with our letrozole baby🤍

People that are suggesting things to get labor started always say nipple stimulation. Is that like using a pump or massaging your nipples, or squeezing them? I don’t know! And how does it work? Does it get contractions going with causes the cervix to thin or dilate? I just need answers! lol